Electrochemical batteries, such as lithium ion secondary batteries, include a separator that is interposed between a positive electrode and a negative electrode. This separator has the function of electrically insulating the positive electrode from the negative electrode, as well as the function of retaining an electrolyte. Currently, a microporous thin film sheet, composed of polyethylene, is mainly used as the separator in lithium ion secondary batteries.
Further, lithium ion secondary batteries include a wound-type electrode group which is prepared, for example, by laminating and winding the positive electrode, the negative electrode and the separator. Such an electrode group, wound so as to have an oval cross section, is also used in rectangular batteries.
With respect to the winding method, various proposals have been made. For example, it is proposed to use a winding core that is preferable, when the electrodes are thick, for eliminating the influence of gaps created by the edges of the electrodes on the initial winding side (see, e.g., Japanese Laid-Open Patent Publication No. Hei 9-35738).
Also, in order to improve high-rate characteristics and life characteristics, it is proposed to form a porous film, composed of a filler and a binder, on the surface of at least one of the positive electrode and the negative electrode, instead of the conventional structure of providing a separator between the positive electrode and the negative electrode (see, e.g., Japanese Laid-Open Patent Publication No. Hei 10-106530 (claim 15 and FIG. 2)).
Such an electrode group is described below with reference to FIG. 5.
An electrode group 50 of FIG. 5 includes a winding core 1, a negative electrode 2, a positive electrode 3, and a porous film 4. The winding core 1 consists of an upper winding core 1a and a lower winding core 1b. The negative electrode 2 comprises a negative electrode core member 2a and a negative electrode active material layer 2b formed on each side of the negative electrode core member 2a. Likewise, the positive electrode 3 comprises a positive electrode core member 3a and a positive electrode active material layer 3b formed on each side of the positive electrode core member 3a. The porous film 4 is formed on the surfaces of the negative electrode active material layers 2b. 
This electrode group is produced as follows. First, the edge of the core member of the negative electrode 2 on the initial winding side is exposed, and the exposed part 5 of the core member is sandwiched between the upper winding core 1a and the lower winding core 1b. Then, the negative electrode 2 and the positive electrode 3 are wound around the winding core 1, with the positive electrode 3 between the negative electrode 2.
In the electrode group of FIG. 5, an independent separator is not provided between the negative electrode and the positive electrode, and the porous film is bonded to the surfaces of at least one of the positive electrode and the negative electrode. Thus, there is no need to prepare an independent separator, so that the structure of the electrode group can be simplified. Further, since the porous film is integrated into the electrode, it is also possible to prevent a short-circuit between the positive electrode and the negative electrode resulting from the displacement of a separator.
Also, a separator can be used together in the electrode group as shown in FIG. 5. FIG. 6 shows an electrode group 60 having a separator. In FIG. 6, the same constituent elements as those in FIG. 5 are given the same numbers.
In the electrode group 60 of FIG. 6, the negative electrode 2 has a separator 6 on each side thereof, i.e., on each of the porous films 4. The separator 6 extends beyond the initial winding position of the negative electrode 2.
This extended part 6a of the separator 6 is sandwiched between the upper winding core 1a and the lower winding core 1b. Then, the negative electrode 2 and the positive electrode 3 are wound around the winding core 1, with the positive electrode 3 between the negative electrode 2, to produce an electrode group.